This invention relates to a method of controlling downhole pressure while drilling through underground formations, and in particular to a method of dynamically controlling the bottom hole circulating pressure in a wellbore passing through a high pressure underground formation. One specific aspect of the invention relates to the drilling of high pressure underground hydrocarbon formations, such as high pressure gas and oil wells.
A common method of drilling wells from the surface through underground formations employs the use of a drill bit that is rotated by means of a downhole motor (sometimes referred to as a mud motor), through rotation of a drill string from the surface, or through a combination of both surface and downhole drive means. Where a downhole motor is utilized, typically energy is transferred from the surface to the downhole motor through pumping a drilling fluid or xe2x80x9cmudxe2x80x9d down through a drill string and channeling the fluid through the motor in order to cause the rotor of the downhole motor to rotate and drive the rotary drill bit. The drilling fluid or mud serves the further function of entraining drill cuttings and circulating them to the surface for removal from the wellbore. In some instances the drilling fluid may also help to lubricate and cool the downhole drilling components.
When drilling for oil and gas there are many instances where the underground formations that are encountered contain hydrocarbons that are subjected to very high pressures. Traditionally, when drilling into such formations a high density drilling fluid or mud is utilized in order to provide a high hydrostatic pressure within the wellbore to counteract the high pressure of the hydrocarbons in the formation below. In such cases the high density of the column of drilling mud exerts a hydrostatic pressure upon the below ground formation that meets or exceeds the underground hydrocarbon pressure thereby preventing a potential blowout which may otherwise occur. Where the hydrostatic pressure of the drilling mud is approximately the same as the underground hydrocarbon pressure, a state of balanced drilling is achieved. However, due to the potential danger of a blowout in high pressure wells, in most instances an overbalanced situation is desired where the hydrostatic head of the drilling mud exceeds the underground hydrocarbon pressure by a predetermined safety factor. The high density mud and the high hydrostatic head that it creates also helps prevent a blowout in the event that a sudden fluid influx or xe2x80x9ckickxe2x80x9d is experienced when drilling through a particular aspect of an underground formation that is under very high pressure, or when first entering a high pressure zone.
Unfortunately, such prior systems that employ high density drilling muds to counterbalance the effects of high pressure underground hydrocarbon deposits have met with only limited success. In order to create a sufficient hydrostatic head in many instances the density of the drilling muds has to be relatively high (for example from 15 to 25 pounds per gallon) necessitating the use of costly density enhancing additives. Such additives not only significantly increase the cost of the drilling operations, but can also present environmental difficulties in terms of their handling and disposal. High density muds are also generally not compatible with many 4-phase surface separation systems that are designed to separate gases, liquids and solids. In typical surface separation systems the high density solids are removed preferentially to the drilled solids and the mud must be re-weighted to ensure that the desired density is maintained before it can be pumped back into the well.
High density drilling muds also present an increased potential for plugging downhole components, particularly where the drilling operation is unintentionally suspended due to mechanical failure. Further, the expense associated with costly high density muds is often increased through their loss into the underground formation. Often the high hydrostatic pressure created by the column of drilling mud in the string results in a portion of the mud being driven into the formation requiring additional fresh mud to be continually added at the surface. Invasion of the drilling mud into the subsurface formation may also cause damage to the formation.
A further limitation of such prior systems involves the degree and level of control that may be exercised over the well. The hydrostatic pressure applied to the bottom of the wellbore is primarily a function of the density of the mud and the depth of the well. For that reason there is only a limited ability to alter the hydrostatic pressure applied to the formation when using high density drilling muds. Generally, varying the hydrostatic pressure requires an alteration of either the density of the drilling mud or the surface backpressure, both of which can be a difficult and time consuming process.
The invention therefore provides a method of dynamically controlling the bottom hole pressure in a high pressure well that addresses a number of limitations in the prior art. In particular, the method of the present invention provides a means to alter and control bottom hole pressure without the need for the utilization of high density, expensive, drilling muds, while also providing a simpler and more time responsive manner to control downhole pressures to react to changing downhole drilling environments.
Accordingly, in one of its aspects the invention provides a method of drilling a well through an underground formation, the method comprising the steps of: with a drill bit drilling a borehole from a location near the surface into the earth; using a first string to define an inner annulus within said borehole, said inner annulus running from the surface to a point proximate the bottom of said borehole; positioning a second string within the borehole about said first string and thereby defining a second annulus between the interior of said second string and the exterior of said first string, thereby also defining an outer annulus exterior to said second string; providing a connecting passageway between said outer annulus and said second annulus at a point uphole from the bottom of said first string, said outer annulus sealed at a point downhole of said connecting passageway such that fluid entering said outer annulus is prevented from escaping into the bottom of the well and is directed through said connecting passageway; providing a supply of pressurized drilling fluid to the drill bit by pumping said drilling fluid through said inner annulus, said drilling fluid flushing cuttings produced by said drill bit through said second annulus and exiting out of said well in the form of drilling fluid returns; and, providing a supply of pressurized fluid to said second annulus by pumping said fluid into said outer annulus and forcing said fluid into said second annulus through said connecting passageway, said fluid forced into said second annulus increasing the friction of said returns flowing through said second annulus resulting in an increase in friction pressure within said second annulus and thereby increasing the bottom hole circulating pressure in the well.
In a further aspect the invention provides a method of drilling an encased well into a high pressure underground hydrocarbon formation utilizing a drill bit drilling a borehole from a location near the surface into the underground formation, the method comprising the steps of: with a first string situated within the borehole, defining an inner annulus running from the surface to a point proximate the bottom of the borehole; placing a second string within the borehole about said first string thereby defining a second annulus between the interior of said second string and the exterior of said first string, thereby also defining an outer annulus between the exterior of said second string and the interior of the well casing; providing a connecting passageway between said outer annulus and said second annulus at a point uphole from the bottom of said first string; providing a supply of pressurized drilling fluid to the drill bit by pumping said drilling fluid through said inner annulus, said drilling fluid flushing cuttings produced by said drill bit through said second annulus, said drilling fluid and said cuttings in said second annulus comprising drilling fluid returns; providing a supply of pressurized fluid to said second annulus by pumping said fluid into said outer annulus and forcing said fluid into said second annulus through said connecting passageway; and, maintaining the bottom hole circulating pressure in the well within defined limits through monitoring the pressure of said returns within said second annulus and controlling the volume and pressure of fluid pumped into said outer annulus in response to fluctuations in the pressure of said returns in said second annulus.
In yet a further embodiment the invention provides a method of drilling an encased well into a high pressure underground hydrocarbon formation utilizing a drill bit to drill a borehole from a location near the surface into the underground formation, the method comprising the steps of: with a first string situated within the borehole, defining an inner annulus running from the surface to a point proximate the bottom of the borehole; placing a second string within the borehole about said first string thereby defining a second annulus between the interior of said second string and the exterior of said first string, thereby also defining an outer annulus between the exterior of said second string and the interior of the well casing; providing a connecting passageway between said outer annulus and said second annulus at a point uphole from the bottom of said first string; providing a supply of pressurized drilling fluid to the drill bit by pumping said drilling fluid through said inner annulus, said drilling fluid flushing cuttings produced by said drill bit through said second annulus, said drilling fluid and said cuttings in said second annulus comprising drilling fluid returns; providing a supply of pressurized fluid to said second annulus by pumping said fluid into said outer annulus and forcing said fluid into said second annulus through said connecting passageway; and, maintaining the bottom hole circulating pressure in the well within defined limits through monitoring the downhole fluid pressure proximate the bottom of the well and controlling the volume and pressure of fluid pumped into said outer annulus in response to fluctuations in the downhole fluid pressure.
In still a further embodiment the invention provides a method of controlling the bottom hole circulating pressure when drilling an encased well through a pressurized underground formation where a supply of pressurized drilling fluid is pumped down an inner annulus in a drill string and released into the bottom of the well to entrain cuttings and flush the cuttings from the well through an outer annulus defined by the exterior of the drill string and the interior of the well casing, the method comprising designing and constructing the drilling system, said drilling system including the drilling fluid, the drill string and the well casing, such that there is sufficient friction pressure generated in said outer annulus when said drilling fluid and said cuttings pass therethrough to create sufficient fluid back pressure at the bottom of the well and to thereby maintain the bottom hole circulating pressure within a desired range for a predetermined drilling fluid flow rate.
The invention also provides a method of drilling a well having a first tubular member extending from the surface of said well to a position proximate the bottom of said well, said first tubular member having an inner annulus, said well also having a second tubular member extending from the surface of said well to a position proximate the bottom of said well, said first and second tubular members forming a second annulus therebetween, said second tubular member and said well forming an outer annulus therebetween, said method comprising pumping a fluid through said inner annulus; and, pumping a fluid through said outer annulus and into said second annulus to control the circulating pressure while drilling said well.
In addition, the invention provides a method of controlling the bottom hole circulating pressure when drilling a well having first and second tubular members extending from the surface of said well to positions proximate the bottom of said well, at least a substantial portion of said first tubular member received within said second tubular member, said first tubular member defining an inner annulus, a second annulus formed between said first and said second tubular members, and an outer annulus formed between said second tubular member and said well, said outer annulus and said second annulus connected by at least one connecting passageway, the method comprising the steps of pumping a fluid into said well through said inner annulus, said fluid flushing drilling cuttings through said second annulus and out of said well; and, pumping a fluid through said outer annulus and through said connecting passageway into said second annulus to control the bottom hole circulating pressure while drilling said well.
In a further aspect the invention provides a method of controlling the bottom hole circulating pressure when drilling a well having first and second tubular members extending from the surface into said well, said well having an inner annulus defined by the interior of said first tubular member, said well having a second annulus defined by the outer surfaces of said first and said second tubular members and the inner surface of said well, said well having an outer annulus defined by the interior of said second tubular member, the method comprising the steps of pumping a fluid through said inner annulus in said first tubular member; and, pumping a fluid through said outer annulus in said second tubular member and into said second annulus to control the circulating pressure while drilling said well.
The invention also provides a method of controlling the bottom hole circulating pressure when drilling an encased well having a first tubular member extending from the surface into said well, said well having an inner annulus defined by the interior of said first tubular member, said well having a second annulus defined by the outer surface of said first tubular member and the inner surface of said well, said well having an outer annulus defined by the interior of a second tubular member extending from the surface along the exterior surface of the well casing, said second tubular member intersecting said well casing at a defined position along the length of said well casing and said outer annulus in communication with said second annulus adjacent said point of intersection, the method comprising the steps of pumping a fluid through said inner annulus in said first tubular member; and, pumping a fluid through said outer annulus in said second tubular member and into said second annulus to control the circulating pressure while drilling said well.
The invention still further provides a method of drilling a well having a first tubular member extending from the surface of said well to a position proximate the bottom of said well, said first tubular member having an inner annulus therethrough, said method comprising pumping a fluid into said well through said inner annulus, said fluid flushing drilling cuttings out of said well; and, injecting a fluid into said well, exterior to said inner annulus, to control the bottom hole circulating pressure in said well.
Further advantages of the invention will become apparent from the following description taken together with the accompanying drawings.